Immunity System

• Antigens are molecules (proteins) on the surface of cells, which enables the immune system to identify: pathogens, abnormal body cells, toxins and cells from other organisms from the same species.
• Phagocytes

  • white blood cells that are produced continuously in the bone marrow
  • 2 main types: neutrophils, macrophages
  • carries out phagocytosis
• Phagocytosis:

  1. Chemicals released by pathogens attract phagocytes (chemotaxis)
  2. Phagocyte recognises foreign antigens
  3. Cytoplasm moves around pathogen, engulfing and trapping it in a phagocytic vacuole (endocytosis)
  4. Lysosome fuses with phagocytic vacuole, releasing lysozymes to digest the pathogen
  5. Phagocyte presents the pathogen’s antigens on its surface
• Cell-mediated response:

  • T-cells are activated when they encounter and bind to their specific antigen that is being presented by a macrophage
  • T-cells divide rapidly by mitosis (clonal expansion) and differentiate into helper T-cells and cytotoxic T-cells
• Helper T-cells:

  • release cytokines which stimulate:
  • the activation of cytotoxic T-cells, which destroy virus infected cells and tumour cells
  • phagocytosis
  • the activation of B-cells
• Cytotoxic T-cells:

  1. Infected cell displays foreign antigens
  2. T-cells punch holes in the cell membrane using perforin
  3. Toxins are secreted by T-cell into the infected cell which kills it
• Humoral response:

  • B-cells have antibody receptors on its cell membrane, and is activated when it binds to a complementary antigen (clonal selection)
  • B-cells divide by mitosis into plasma cells and memory cells (clonal expansion)
• Plasma cells:

  • secrete antibodies specific to the antigen (monoclonal antibodies), which binds to form antigen-antibody complexes
  • short-lived
• Structure of antibodies:

  • quaternary structure, with 2 heavy polypeptide chains bonded to 2 light chains by disulfide bonds
  • each chain has a constant region and variable region
  • variable region is where the antibody attaches to antigens, its amino acid sequence is different for each antibody
  • hinge region gives flexibility, allowing antigen-binding site to be placed at different angles
• • Antibodies have 2 antigen-binding sites, so they can bind to 2 pathogens at once
  • This allows pathogens to be clumped together (agglutination)
  • Phagocytes can engulf and destroy many pathogens at once
• Primary immune response:

  • After clonal selection and clonal expansion of B-cells, plasma cells secrete lots of antibodies to fight the pathogen
  • Memory cells are produced - T-cells remember antigen, B-cells remember antibodies needed
  • Plasma cells are short-lived while memory cells are long-lasting
• Secondary immune response:

  • if the same antigen is found in the body, memory cells recognise the antigen and divide into plasma cells that produce the right antibody to the antigen
  • quicker response compared to primary immune response